Title Snow density along the route traversed by the Japanese-Swedish Antarctic Expedition 2007/08 Author(s) Sugiyama, Shin; Enomoto, Hiroyuki; Fujita, Shuji; Fukui, Kotaro; Nakazawa, Fumio; Holmlund, Per; Surdyk, Sylviane Journal of Glaciology, 58(209), 529-539 Citation https://doi.org/10.3189/2012JoG11J201 Issue Date 2012-06 Doc URL http://hdl.handle.net/2115/50785 Rights © 2012 International Glaciological Society Type article File Information JoG58-209_529-539.pdf Instructions for use Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP Journal of Glaciology, Vol. 58, No. 209, 2012 doi: 10.3189/2012JoG11J201 529 Snow density along the route traversed by the Japanese–Swedish Antarctic Expedition 2007/08 Shin SUGIYAMA,1 Hiroyuki ENOMOTO,2 Shuji FUJITA,2 Kotaro FUKUI,3 Fumio NAKAZAWA,2 Per HOLMLUND,4 Sylviane SURDYK2 1Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan E-mail: [email protected] 2National Institute of Polar Research, Tachikawa, Japan 3Tateyama Caldera Sabo Museum, Toyama, Japan 4Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden ABSTRACT. During the Japanese–Swedish Antarctic traverse expedition of 2007/08, we measured the surface snow density at 46 locations along the 2800 km long route from Syowa station to Wasa station in East Antarctica. The mean snow density for the upper 1 (or 0.5) m layer varied from 333 to 439 kg m–3 over a region spanning an elevation range of 365–3800 m a.s.l. The density variations were associated with the elevation of the sampling sites; the density decreased as the elevation increased, moving from the coastal region inland. However, the density was relatively insensitive to the change in elevation along the ridge on the Antarctic plateau between Dome F and Kohnen stations. Because surface wind is weak in this region, irrespective of elevation, the wind speed was suggested to play a key role in the near-surface densification. The results of multiple regression performed on the density using meteorological variables were significantly improved by the inclusion of wind speed as a predictor. The regression analysis yielded a linear dependence between the density and the wind speed, with a coefficient of 13.5 kg m–3 (m s–1)–1. This relationship is nearly three times stronger than a value previously computed from a dataset available in Antarctica. Our data indicate that the wind speed is more important to estimates of the surface snow density in Antarctica than has been previously assumed. 1. INTRODUCTION Since the direct determination of snow density from Knowledge of the near-surface snow density is essential for satellite data remains difficult, firn-core analyses and pit the study of surface processes in the Antarctic ice sheet. For measurements are the only means to obtain reliable data. instance, the snow density is required to calculate the However, it is difficult to cover a large area using point surface mass balance from stake measurements (e.g. measurements, and the measurement sites are not uni- Hubbard and Glasser, 2005; Takahashi and Kameda, 2007) formly distributed throughout Antarctica. To overcome and to interpret satellite/airborne altimetry (e.g. Zwally and these problems, densification models have been coupled Li, 2002; Helsen and others, 2008). The density is also with climate and meteorological models to compute the important for various types of satellite data analyses because density distribution over the entire ice sheet. Density maps it is directly related to the dielectric permittivity of snow (e.g. obtained using these models have contributed to our Tiuri and others, 1984), which controls the emission and understanding of the age of gases contained in ice cores reflection of electromagnetic waves (e.g. Grody, 2008; (Kaspers and others, 2004), firn thickness (Van den Broeke, Lacroix and others, 2009). Further, snow densification 2008) and ice-sheet elevation change (Helsen and others, accompanies changes in the size, structure and bonding of 2008). Such an approach requires an accurate densifi- snow particles, properties which are important for finding cation model calibrated with field data, and the modeled climatic signals in deep ice cores. density map needs to be validated through comparison With these factors as motivation, surface snow density with field observations. and the densification process have been studied by numer- Despite the need for reliable near-surface snow density ous observational (field and satellite), theoretical and data, previously reported field measurements are insufficient numerical methods. The snow density has been measured to cover the large variability in meteorological and geo- at many locations in Antarctica (e.g. Cameron and others, metrical conditions in Antarctica. Data are particularly 1968; Endo and Fujiwara, 1973; Braaten, 1997; Oerter and scarce for the interior of the ice sheet. To contribute to our others, 1999; Eisen and others, 2008; Kameda and others, understanding of the snow density distribution and densifi- 2008). Near-surface densification processes have been cation processes in Antarctica, we carried out shallow pit studied by measuring the vertical density profiles of firn measurements during the Japanese–Swedish traverse expe- cores and shallow pits (e.g. Alley, 1988; Craven and Allison, dition in the 2007/08 austral summer. In this paper, we 1998; Van den Broeke and others, 1999; Fujita and others, report upper 1 (or 0.5) m density profiles collected at 2009; Vihma and others, 2011). These field data have been 46 survey sites, from coastal to inland plateau areas in East used to develop and calibrate numerous snow densification Antarctica. The influence of meteorological variables on models (e.g. Herron and Langway, 1980; Kameda and near-surface densification is discussed in light of the density others, 1994; Spencer and others, 2001). variation measured along the 2800 km long route. 530 Sugiyama and others: Snow density along the JASE traverse route Fig. 1. Map of the study region, along with the route traversed by the Japanese–Swedish Antarctic Expedition in 2007/08. Open circles denote the locations of snow-pit measurements. Red circles denote measurement sites referred to in the text. The contours represent surface elevation at intervals of 200 m, based on Bamber and others (2009). 2. METHOD 28 January 2008. The elevation of the measurement sites 2.1. Study site increased from 1991 m (Z8) to 3800 m (Dome F), then gradually decreased to 2500 m at the point 290 km west of From November 2007 to January 2008, Japanese and Kohnen station. From this point, the route descended steeply Swedish research teams carried out a traverse expedition from the Antarctic plateau to the coastal region. Annual in East Antarctica, between the Japanese inland base S16 snow layer thicknesses near the surface have previously (30 km from Syowa station; 69.038 S, 40.058 E; 589 m a.s.l.) 8 8 been measured at 50–400 mm for the section between Z8 and the Swedish Wasa station (73.05 S, 13.37 W; 292 m and Dome F (Furukawa and others, 1996), 140–250 mm in a.s.l.) (Fig. 1; Table 1) (Holmlund and Fujita, 2009). The two the vicinity of Kohnen (Oerter and others, 1999) and teams departed from their home stations in snow vehicles 550 Æ 110 mm for the coastal region near Wasa (Ka¨rka¨s and met at a point 1400 km from each of the starting points 8 8 and others, 2005). The uppermost 1 m of snow cover (75.89 S, 25.83 E; 3661 m a.s.l.). They exchanged some therefore consists of several annual layers, in some cases expedition members and scientific instruments before they more than ten. began their return trips. Accordingly, various research activities could be carried out along the entire stretch from 2.2. Measurements S16 to Wasa (Sugiyama and others, 2010; Fujita and others, 2011). Snow measurements were performed on a side-wall of a During the expedition, we measured the density, grain snow pit excavated at each survey site. We measured the size and structure of snow in the near-surface layer at density by sampling a 30 mm thick snow block using a box- Â Â 46 locations along the route (to 1 m depth at 35 locations type stainless density cutter (30 mm 60 mm 56 mm, and to 0.5 m at 11 locations) (Fig. 1). The first survey was Climate Engineering Co.). A digital bench scale (CS200, Æ made at Z8 (180 km inland from S16) on 17 November OHAUS Co.) with a resolution of 0.1 g was used to weigh 2007. The snow survey was repeated along the rest of the the snow blocks. This error corresponds to <1% of measured expedition route (covering >2650 km), with measurements weight. The accuracy for density measurements performed taken at intervals of approximately 20–100 km until the final with similar devices and procedures has been reported as Æ measurement was completed at AWS5 near Wasa station, on 4% (Conger and McClung, 2009). The sampling was performed from the surface to 1 (or 0.5) m depth, at intervals of 30 mm. Stratigraphical information was recorded following the Table 1. Sites along the expedition route referred to in the text guidelines given by Colbeck and others (1990). We identified layers after a visual inspection of the snow structure and grain size. The snow structure was classified Site Lat. Long. Elevation Distance from S16 into four types: fresh snow; compacted snow (Fig. 2a); mkmfaceted crystals or depth hoar (we refer to both types of structure as depth hoar in the remainder of this paper) S16 69.038 S 40.058 E 589 0 (Fig. 2b); and a crust layer (Fig. 2c). The compacted snow Z8 70.088 S 43.248 E 1991 180 layers include those compacted under overburden stress as 8 8 MD228 72.79 S 43.52 E 2960 495 well as wind compaction.